Field of the Invention
Polyamides, such as poly-.epsilon.-caprolactam and polyhexamethyleneadipamide, are among the engineering plastics which have been known, and are used in many fields. They generally possess great hardness and rigidity and good heat distortion resistance and are also resistant to abrasion and wear and to many chemicals. The toughening of such polyamides is likewise known and is described in, for example, DE-A 26 22 973 and U.S. Pat. No. 3,976,771, to mention but two examples.
For some intended uses, however, it would be desirable for the heat distortion resistance of the polyamides to be further improved without the remaining mechanical properties, in particular the impact strength, being adversely affected, as well as evincing an improved modulus of elasticity, such modulus not being affected by ambient humidity.
Copolyamides in which some of the aliphatic units are replaced by aromatic units have long been known, for example copolyamides of adipic acid, terephthalic acid, hexamethylenediamine and .epsilon.-caprolactam in any combination.
Thus, German Patent 929,151 describes a process for the preparation of highly polymeric linear polyamides, in which a mixture of an aromatic para-dicarboxylic acid or an amide-forming derivative thereof, an equivalent amount of an aliphatic or cycloaliphatic diamine and another polyamide-forming starting material, for example a lactam, is condensed under polyamide-forming conditions.
British Patent 1,114,541 describes ternary copolyamides which, in addition to a major amount of poly-hexamethyleneadipamide, contain 20-40% by weight of units derived from terephthalic acid and hexamethylenediamine and 2-20% by weight of another polyamide component.
German Laid-Open Application DOS 1,669,455 describes a process for the preparation of stretched polyamide filaments by melt-spinning a copolyamide, wherein the copolyamide contains not more than 40% by weight of units derived from terephthalic acid and hexamethylenediamine, and the preparation of these polyamides is carried out in the presence of not less than 3 mol % of a monofunctional acidic or basic stabilizer.
German Laid-Open Application DOS 1,620,997 describes linear fiber-forming terpolyamides which contain units derived from adipic acid and hexamethylenediamine, from terephthalic acid and hexamethylenediamine and from isophthalic acid and hexamethylenediamine.
German Laid-Open Application DOS 3,407,492 describes a process for the preparation of copolyamides from adipic acid, terephthalic acid and hexamethylenediamine, wherein the copolyamide contains 25-48% by weight of units of hexamethyleneterephthalamide and, in the process, a 40-70% strength aqueous solution of the monomers is heated to not less than 250.degree. C. in less than 15 minutes and is condensed to a relative viscosity of 1.5-2.4. Thereafter, the water is distilled off in one or more stages, and the resulting precondensate is subjected to postcondensation in a known manner.
None of the above-mentioned publications contains more detailed information or indications regarding the behavior of the partly aromatic copolyamides described on modification with fibrous or particulate fillers.
During investigations into modification of the partly aromatic copolyamides described above with fillers, we have found that the products have a high melt viscosity, and moldings produced from the materials could only be processed under high filling pressure and had many gel particles and specks, i.e., a poor surface quality. Moreover, owing to the gel particles, the filler distribution was uneven and therefore unsatisfactory.